drm/i915/skl: Register definitions for SKL Clocks

This patch defines the necessary SKL registers for implementing the
new clocking mechanism.

v2: Addressed review comments by Damien
	- Added code comment
	- Introduced enum for WRPLL values

v3: Rebase on top of nightly (minor conflict in i915_reg.h)

v4: Use 0x, not 0X (Ville)

v5: Modified as per review comments from Paulo

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v2)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v3,v4)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>